1. Field of the Invention
The present invention relates to a bulk acoustic wave device using thickness longitudinal vibrations of a piezoelectric thin film, which is applicable in a high-frequency filter or a high-frequency oscillator. More specifically, the present invention relates to a bulk acoustic wave device which is hermetically sealed on a substrate by means of a protection layer, and a method of manufacturing such a bulk acoustic wave device.
2. Related Art
A bulk acoustic wave (BAW) element, or a thin film bulk acoustic wave resonator (so called FBAR), using thickness longitudinal vibrations of a piezoelectric layer is regarded as a promising element to be applied to an RF filter in mobile radio communication or a voltage-controlled oscillator, since with such an element, it is possible to obtain a high excitation efficiency and a sharp resonance characteristic in a frequency band on the order of GHz or higher, with very small dimensions.
In a bulk acoustic wave device, the resonance frequency is determined based on the degree of acoustic velocity and thickness of piezoelectric material used. Generally, a thickness of 1 μm-2 μm corresponds to 2 GHz, and a thickness of 0.4 μm-0.8 μm corresponds to 5 GHz. In this manner, it is possible to cope with a high frequency up to several tens of GHz.
It is possible to form an RF filter of a mobile communication device with a plurality of bulk acoustic wave devices including the aforementioned bulk acoustic wave elements by arranging the bulk acoustic wave devices in series and in parallel to form a ladder filter as shown in FIG. 10. Furthermore, it is possible to form a voltage controlled oscillator (VCO) of a mobile communication device by combining a bulk acoustic wave device, a variable capacitance diode, and an amplifier, as shown in FIG. 11.
The performance of a bulk acoustic wave element can be shown by an electromechanical coupling factor kt2 and a quality factor Q. As the electromechanical coupling factor becomes higher, a filter or VCO having a wider band can be formed. In order to increase the electromechanical coupling factor, a material having a higher electromechanical coupling factor, which is inherent to its crystal, should be used, and the polarization axis of crystal should be aligned along the thickness direction of the layer. The quality factor Q relates to an insertion loss at the time a filter is formed, or the accuracy of the oscillations from an oscillator. Various phenomena to absorb elastic waves are related to Q, and it is possible to obtain a larger Q by improving the purity of crystal, aligning the crystal orientation, and using a piezoelectric layer having aligned polarization.
For example, Japanese Patent Laid-Open Publication No. 2000-69594 discloses a conventional and typical bulk acoustic wave device. FIGS. 12-16 show the structure and the method of manufacturing this bulk acoustic wave device. First, as shown in FIG. 12, a recess 52 is formed on a silicon substrate 51 by anisotropic etching, and then the surface of the silicon substrate 51 is covered by an insulating layer 53. Thereafter, a hollow section forming layer 55, which is easy to etch, e.g., a silicate glass to which boron or phosphorus is doped (boron-phosphorus silicate glass=BPSG), is formed on the substrate 51, as shown in FIG. 13. Subsequently, the workpiece is polished until the surface of the insulating layer 53 is exposed, and then the surface is flattened. As a result, the hollow section forming layer 55 is left only in the recess 52, as shown in FIG. 14.
Then, a lower electrode layer, a piezoelectric layer, and an upper electrode layer are sequentially deposited and patterned, thereby forming a bulk acoustic wave element 60 including a lower electrode 60b, a piezoelectric layer 60a, and an upper electrode 60c formed on the hollow section forming layer 55, as shown in FIG. 15.
Subsequently, a through-hole (not shown) reaching the hollow section forming layer 55 is formed through the bulk acoustic wave element 60, thereby removing the hollow section forming layer 55 by selective etching, as shown in FIG. 16. Through this process, the bulk acoustic wave element is formed.
The resonating part of the bulk acoustic wave element 60, composed of the piezoelectric layer 60a and the upper and lower electrodes 60b and 60c, should be hermetically sealed in a package formed of alumina, etc., since it is necessary to sandwich the resonating part with air layers in order to trap vibration energy. FIG. 17 shows an example of such a hermetically-sealed package. A bulk acoustic wave element 60 is connected to an alumina substrate 71 with wire bonding 73, and then the substrate 71 is connected to a cover 77 formed of alumina with solder 75, thereby hermetically sealing the bulk acoustic wave element 60.
As described above, it is necessary for the conventional bulk acoustic wave device to have air layers above the upper electrode 60c and below the lower electrode 60b in order to trap vibration energy between the upper and lower electrodes. In addition, it is necessary to seal the entire device in order to protect the electrode layers, etc. from the outside environment. The air layer below the lower electrode 60b can be formed by first making a hollow section forming layer 55 and then removing it by selective etching, as shown in FIGS. 12 to 16. The air layer above the upper electrode 60c is formed, however, by sealing the workpiece in the package of alumina, etc., as shown in FIG. 17. Since the structure of the package is complicated, the entire cost tends to increase. In addition, there is a problem in that the size of the package becomes rather large.